1. Field
The invention relates to a fuel rod cladding tube made out of silicon carbide (herein referred as SiC), regardless the cladding design architecture (monolithic, duplex with monolithic SiC on the inside and a composite made with SiC fibers and SiC matrix on the outside, etc.) with a spark plasma sinter sealed end plug.
2. Description of Related Art
In a typical nuclear reactor, such as a pressurized water (PWR), heavy water (such as a CANDU) or a boiling water reactor (BWR), the reactor core includes a large number of fuel assemblies, each of which is composed of a plurality of elongated fuel elements or fuel rods. The fuel rods each contain nuclear fuel fissile material such as at least one of uranium dioxide (UO2), plutonium dioxide (PuO2), uranium nitride (UN) and/or uranium silicide (U3Si2); with possible additions of, for example, boron or boron compounds, gadolinium or gadolinium compounds and the like either on or in pellets, usually in the form of a stack of nuclear fuel pellets, although annular or particle forms of fuel are also used. The fuel rods have a cladding that acts as a containment for the fissile material. The fuel rods are grouped together in an array which is organized to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A coolant, such as water, is pumped through the core in order to extract the heat generated in the core for the production of useful work. Fuel assemblies vary in size and design depending on the desired size of the core and the size of the reactor.
The cladding on the fuel rods is usually made from zirconium (Zr) with up to about 2 wt. % of other metals such as Nb, Sn, Fe and Cr. Such zirconium alloy clad tubes are taught, for example, by Biancheria et al., Kapil and Lahoda (U.S. Pat. Nos. 3,427,222; 5,075,075; and 7,139,360, respectively). The fuel rods/cladding have an end cap at each end and a hold down device such as a metal spring to keep the stack of nuclear fuel pellets in place. FIG. 1 illustrates this type of prior art design, showing a string of fuel pellets 10, a zirconium-based cladding 12, a spring holdown device 14, and end caps 16.
There are problems associated with metal clad fuel rods. They can wear if contacted by debris that may be present in the cooling water mentioned before. Under severe conditions such as “beyond design basis” accidents; metal cladding can react exothermally with steam at over 1,093° C. (2,000° F.). These zirconium cladding metals protecting the nuclear fuel may lose strength during “a loss of coolant” accident, where reactor temperatures can reach as high as 1,204° C. (2,200° F.), and expand due to internal fission gases within the fuel rod. In addition, continuing utility industry demands have pushed reactor operating temperatures and cladding radiation exposure to extreme limits.
All this has prompted considering use of experimental ceramic type materials such as silicon carbide (SiC) monolith, fibers and their combinations as taught by Maruyama et al. (U.S. Pat. No. 6,246,740), Zender, (U.S. Pat. No. 5,391,428), Hanzawa et al., (U.S. Pat. No. 5,338,576); Feinroth (U.S. Pat. No. 5,182,077 and U.S. Patent Publication No. 2006/0039524 A1), Easier et al. (U.S. Patent Publication No. 2007/0189952 A1); and tangentially Korton, (U.S. Pat. No. 6,697,448) as complete or partial substitutes for metal fuel rods.
The absolutely right combination must be sought in the nuclear industry to make usually brittle ceramic, much more flexible, to relieve stress/temperature/pressure in full failure conditions. One possibility is use of experimental SiC fiber reinforced SiC composites; a two or three-layer tube of high purity beta or alpha phase stoichiometric silicon carbide covered by a central composite layer of continuous beta phase stoichiometric silicon carbide fibers infiltrated with beta phase SiC and, in the case of three layers, an outer protective layer of fine grained beta phase silicon carbide. It has been suggested to pre-stress the fiber component, forming the fibers into tows and tow reverse winding overlapping; where the fibers are coated with a less than a micrometer of SiC or carbon or graphite or boron nitride to provide a weak interface allowing slippage, all this to get better strain resistance and flexibility. Feinroth et al. in U.S. Patent Publication No. 2006/0039524 A1, herein incorporated by reference, describes such nuclear fuel tubes and a matrix densification using well known processes of chemical vapor infiltration (CVI), polymer impregnation, and pyrolysis (PIP). Alumina (Al2O3) fibers in an alumina matrix have also been suggested as a substitute.
As used herein, the term “Ceramic Composite” will mean and is defined as all of the above described composite type structures including SiC and Al2O3.
Surprisingly, very little is said about the end plugs for such ceramic composites. In fact, finding a sealing technology that attaches an end plug, and ensures hermeticity for a Ceramic Composite cladding, such as a silicon-carbide fuel rod cladding has been a very elusive task so far, due to the various requirements placed on such an interface joint that is to:                ensure mechanical strength during and after normal operation, anticipated operational occurrences, infrequent accidents, and limiting faults;        ensure the hermeticity of the end plug-to-cladding joint under irradiation and the nuclear reactor-specific corrosive environment;        allow the joining process to accommodate fully-loaded cladding (with fuel pellets and a hold down device). In addition, the end plug and sealing technology must allow for pressurization of the fuel rod with Helium or other thermally conductive backfill gas at pressures typically up to 300 psi; and        allow the joining process to accommodate economy-of-scale commercial use.        
Several sealing technologies have been explored in the recent past; but to date none of them proved successful in a nuclear environment, which is essential here. Thus, there are a number of sealing technologies suggested, that use various compounds (other than SiC) to seal SiC parts (e.g., Ti-based formulations, Al—Si formulations), including brazing and other techniques, for example: V. Chaumat et al, U.S. Patent Publication No. 2013/0004325A1; A. Gasse, U.S. Patent Publication 2003/0038166; A. Gasse et al., U.S. Pat. No. 5,975,407; F. Montgomery et al., (U.S. Pat. No. 5,447,683); G. A. Rossi et al., (U.S. Pat. No. 4,925,608); and McDermid, “Thermodynamic brazing alloy design for joining silicon carbide,” J. Am. Ceram. Soc., Vol. 74, No. 8, pp. 1855-1860, 1991.
There has been an explosion of research on SiC joining technology since 2007; for example: C. H. Henager, Jr. et al., “Coatings and joining for SiC and SiC composites for nuclear energy systems,” Journal of Nuclear Materials, 367, 370 (2007) 1139-1143; M. Ferraris et al., “Joining of machined SiC/SiC composites for thermonuclear fusion reactors,” Journal of Nuclear Materials, 375 (2008) 410-415; J. Li et al., “A high temperature Ti—Si eutectic braze for joining SiC,” Materials Letters, 62 (2008), 3135-3138; W. Tian, “Reaction joining of SiC ceramics using TiB2-based composites,” Journal of the European Ceramic Society, 30 (2010) 3203-3208 and M. Ferraris et al., “Joining of SiC-based materials for nuclear energy applications,” Journal of Nuclear Materials, 417 (2011) 379-382. These articles are attempting to apprise utilities, of means to ensure higher and higher outputs. The utilities are requiring more and more stressed designs and materials as is economically necessary to meet world energy needs.
The above ceramic models are no longer experiments and are generally shown to have high mechanical strength, and are thought capable of realizing the required gas-tightness for a nuclear reactor; however, these joining technologies failed to show the corrosion and irradiation resistance necessary to survive in a nuclear reactor environment for a typical lifetime of a fuel rod. Other sealing technologies (e.g., experimental spark plasma sintering—hereinafter “SPS”) described by Munir et al., “The effect of electric field and pressure on the synthesis and consolidation of materials herein incorporated by reference, describes: a review of the spark plasma sintering method,” J. Mater Sci., 41 (2006) 763, 777. They make no use of additional chemical compounds; but economical large-scale manufacturing using this process are illusive to this date and still remains a challenge. Hot isostacic pressure (HIP), a well known technique for use in many commercial areas, can also be used to join SiC to SiC; but HIP, noted previously, A Rossi et al., U.S. Pat. No. 4,925,608, is not practical in the fragile environment of sealing nuclear fuel rods due to the long sintering cycle and high temperatures, about 1,700° C., and extremely high pressure, and is not applicable to mass production. What is needed is a commercially viable joining method for sealing tubular ceramic composites with end caps of ceramic or metal.
It is a main object of this invention to provide a method for producing high strength, hermetically sealed, commercially useful and viable end plug seals and methods, resistant to irradiation in a dramatic nuclear environment using ceramic composite tubes as a basis for containing the fuel pellets.